Aug. 21, 2013, provided by Shaam News Network, which has been authenticated based on its contents and other AP reporting, purports to show several bodies being buried during a funeral in a suburb of Damascus, Syria.

WASHINGTON — A United Nations report released Monday confirmed that poison gas was used against Syrian civilians in a deadly attack last month that U.N. Secretary-General Ban Ki-moon called the most significant use of chemical weapons against civilians since Saddam Hussein gassed thousands of villagers in 1988.

Ban said U.N. inspectors had found overwhelming and indisputable evidence that surface-to-surface rockets carrying the banned nerve agent sarin were fired into the Ghouta suburb of Damascus on Aug. 21, according to the report presented to the U.N. Security Council. Assigning culpability for the attack wasnt part of the teams mandate, but experts say the findings point to Syrian President Bashar Assads forces based on the munitions used and the sophistication and range of the delivery system.

Any way you cut it, this has been a bad day for Bashar Assad, said Brian Finlay, a chemical weapons specialist whos the managing director of the Stimson Center, a Washington-based global security research center. The report suggests he either A, ordered chemical weapons use, or B, lost control of his military and they used it.

In New York, Ban refused to speculate on blame, but he called the attack a war crime and vowed that the perpetrators would be held accountable. He added that issues of responsibility would be discussed at the Security Council, where talks are underway over potential consequences should Assad fail to comply with a U.S.-Russian plan to seize Syrian chemical stockpiles.

We may all have our own thoughts on this, but I will simply say that this was a grave crime and those responsible must be brought to justice as soon as possible, Ban said.

The report, considered the most thorough and neutral to date in the Syrian chemical weapons controversy, concludes that chemical weapons were used on a relatively large scale in the civil war thats raged for more than two years. The Ghouta attack was particularly deadly, the report said, because it took place very early in the morning, when low temperatures helped the gas seep into basements where families had taken shelter.

Just getting to the impact sites was a feat for the inspectors, who relied on assurances from both the regime and rebel fighters for safe passage but still came under fire and received repeated threats of harm. With only hours to conduct their work, team members interviewed more than 50 survivors, medical personnel and first responders, according to the U.N. They assessed victims symptoms and collected hair, urine and blood samples.

The survivors accounts make for what Ban called chilling reading, with descriptions of common symptoms suffered immediately after the shelling that morning: shortness of breath, runny noses, blurred vision, vomiting, weakness and eventual loss of consciousness. The report said people who first went to the scene to help the victims observed a large number of individuals lying on the ground, dead or unconscious, and then began suffering their own symptoms.

Several of these first responders also became ill, with one describing the onset of blurred vision, generalized weakness, shaking, a sensation of impending doom, followed by fainting, according to the report.

Under whats described as a meticulously guarded process to protect the chain of custody, the samples were sent to four laboratories for testing. Eighty-five percent of the blood samples tested positive for sarin, according to the findings. Put another way, of 34 patients who had signs of poisoning, almost all tested positive for exposure to sarin.

The British ambassador to the United Nations said after he was briefed on the report that the head of the investigation, Dr. Ake Sellstrom, had described the sarin in the assault as of better quality than that used in a 1995 Tokyo terrorist attack or in an Iraqi attack on Kurdish villages in 1988.

This was no cottage industry use of chemical weapons, he said, adding that it confirmed the British governments belief that only Assads military could have launched the attack.

The U.N. team also examined impact sites and munitions, and collected 30 soil and environmental samples, far more than any previous such United Nations investigation. A majority of the rockets or rocket fragments the team examined were confirmed to contain sarin.

The U.N., Human Rights Watch and a number of independent arms-control monitors have determined that the chemical weapons attack in question was delivered by a combination of two unguided surface-to-surface rocket systems  140 mm and 330 mm  both of which are known to be in the arsenals of Iran, Syria and the militant group Hezbollah but which havent been claimed or alleged to be in the hands of the rebels, who would find such weapons useful in the siege-style battles common to the civil war.

Without directly pointing the finger, the report left little doubt that the rockets carrying the sarin payload were fired from Syrian government territory and that at least some of them were of Russian manufacture. A photograph in the report showed Cyrillic lettering on the engine of a 140 mm rocket that the inspectors had examined.

Eliot Higgins, a British blogger who writes on the weapons used in the Syrian civil war, wrote on his Brown Moses blog that the evidence for the Syrian governments responsibility far outweighed suggestions of rebel culpability. Recounting the details of the two types of rockets found  one of likely Russian manufacture, the other possibly of Iranian origin  Higgins noted that the rebels have never been known to use either. He also pointed out that rockets similar to the possible Iranian rocket were found at the site of an Aug. 5 alleged chemical attack.

He said the warheads of similar rockets photographed at the scenes of other attacks all had split open the same way, an intentional design feature that was "beyond anything the opposition has manufactured themselves."

The settings on the guidance systems as reported by the inspectors made clear that the rockets were fired from the northeast of Damascus  where the government is in control  to the eastern suburbs, which are under control of rebel forces, said Greg Thielmann, a senior fellow at the Arms Control Association, a nonpartisan research center in Washington.

The advocacy group Human Rights Watch said in a report last week that the Soviet Union had shipped numerous 140 mm rockets and hundreds of rocket launchers to Syria. But Thielmann noted that it wasnt clear whether Syrian engineers had modified the original Soviet design.

The reports disclosures appeared to discredit Russian President Vladimir Putins assertion last week in an opinion column in The New York Times that there is every reason to believe sarin was used not by the Syrian army, but by opposition forces.

You would have thought that the Russians would have known this report was coming, Thielmann said. I dont know why they would have set themselves up this way.

Press Secretary Jay Carney said the White House was reviewing the U.N. report, and he noted that although it didnt specify a source of the attacks, the findings "do support the conclusion the world already reached based on overwhelming evidence that the Assad regime was responsible for the chemical weapons attack on Aug. 21.

He added that only the Assad regime was capable of delivering that attack, both in the means that it was delivered through surface-to-surface rockets, and using the agent that was used.

The Syrian government didnt immediately respond to the report beyond the state news agency linking to a demand by Russian officials that outside experts study the document and its methodology. However, top Syrian official denounced a report last week by a separate U.N. body thats looking into possible war crimes committed by both sides.

Dr. Faisal al Hamwi, a Geneva-based Syrian diplomat, charged that the Commission of Inquiry report on war crimes had relied on lies delivered from fugitives from ongoing terrorism investigations. The regime typically refers to rebel forces as terrorists, and emphasizes the lead role played by al Qaida-style extremists.

The U.N. emphasized that the full picture of chemical weapons use in the Syrian conflict was still incomplete and announced that inspectors would return as soon as they could work out an agreement with the government in order to investigate other sites of purported chemical attacks.

That presumably would include sites of attacks where the suspected agent was chlorine and the delivery system was cruder, which could suggest rebel or other nongovernmental actors, said Finlay, the chemical weapons specialist at the Stimson Center.

Theres enough ambiguity there that one could, if you were sitting in Moscow or Damascus, make a not-unreasonable argument that the Assad regime was not behind those attacks, Finlay said.

Lesley Clark and Mark Seibel in Washington and McClatchy special correspondent Mitchell Prothero in Beirut contributed to this article.

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